Aerobic fitness is the strongest correlate of 8-year survival in patients with cystic fibrosis (CF); exercise training has been shown to improve fitness in these children. Because of fluctuations in clinical status and limitations in ventilatory capacity, children with CF need exercise programs regulated by parameters other than target heart rate. The primary aim of this randomized clinical trial is to test the effect of a 6 month program of self-regulated exercise (SRE) with telephone reinforcement on the primary endpoint of cardiorespiratory fitness (measured as peak oxygen uptake [VO2]) of children with CF, compared to attention control (AC). The Children's OMNI Scale of Perceived Exertion will be used to make adjustments in training intensity. The secondary aims are to 1) compare the groups on other endpoints: pulmonary function (FEV1), HRQoL, and additional measures of fitness; 2) evaluate maintenance of peak VO2 in the SRE group after removal of the telephone reinforcement; 3) evaluate the intervention in a "real world" setting by having the AC group cross over to SRE without telephone reinforcement; and 4) explore the exercise experiences of children and parents in both groups. Fifty subjects from the CF Center at Children's Hospital of Pittsburgh will be stratified by peak VO2 and randomly assigned to the self-regulated exercise (SRE) with telephone reinforcement or the attention control group. The SRE group will engage in a 3-times-a-week at-home exercise regimen for 6 months, while the AC group will receive standard care. Both groups will be contacted weekly by telephone during the initial 6 months of the study. Measurements for both groups will occur at baseline and 2, 6, 8, and 12 months. During the second 6 months of the study, the subjects in the SRE with telephone reinforcement group will be instructed to continue their exercise program, but they will not receive telephone reinforcement. The subjects in the AC group will crossover to SRE without telephone reinforcement. The primary analysis will test the difference in peak VO2 by intervention group. Similar analyses will be carried out for the main secondary endpoint, FEV1, and for the HRQoL score, using longitudinal repeated measures models. This innovative self-regulated approach to exercise training may promote long-term maintenance of exercise, holding great promise for improving the quality and perhaps even length of CF patients' lives.